A Bluetooth system provides a communication channel between two electronic devices via a short-range radio link. In particular, the Bluetooth system operates in the radio frequency range around 2.45 GHz in the unlicensed Industrial-Scientific-Medical (ISM) band. The Bluetooth radio link is intended to be a cable replacement between portable and/or fixed electronic devices. The portable devices include mobile phones, communicators, audio is headsets, laptop computers, other GEOS-based or palm OS-based devices and devices with different operating systems.
The Bluetooth operating band is globally available, but the permissible bandwidth of the Bluetooth band and the available RF channels may be different from one country to another. Globally, the Bluetooth operating band falls within the 2400 MHz to 2483.5 MHz range. In the U.S. and in Europe, a band of 83.5 MHz bandwidth is available, and the band is divided into 79 RF channels spaced 1 MHz apart. Bluetooth network arrangements can be either point-to-point or point-to-multipoint to provide connection links among a plurality of electronic devices. Two to eight devices can be operatively connected into a piconet, wherein, at a given period, one of the devices serves as the master while the others are the slaves. Several piconets may form a larger communications network known as a scatternet, with each piconet maintaining its independence. The baseband protocol for a Bluetooth system combines circuit and packet switching. Circuit switching can be either asynchronous or synchronous. Up to three synchronous data (logical) channels, or one synchronous and one asynchronous data channel, can be supported on one physical channel. Each synchronous channel can support a 64 kb/s transfer rate while an asynchronous channel can transmit up to 721 kb/s in one direction and 57.6 kb/s in the opposite direction. If the link is symmetric, the transfer rate in the asynchronous channel can support 432.6 kb/s. A typical Bluetooth system consists of a radio link, a link control unit and a support unit for link management and host terminal interface functions. The Bluetooth link controller carries out the base band protocols and other low-level routines. Link layer messages for link set-up and control are defined in the Link Manager Protocol (LMP). In order to overcome the problems of radio noise interference and signal fading, frequency hopping is currently used to make the connections robust.
Currently, each of the 79 RF channels is utilized by a pseudo-random hopping sequence through the Bluetooth bandwidth. The hopping sequence is unique for each piconet and is determined by the Bluetooth device address of the master whose clock is used to determine the phase of the hopping sequence. The channel is divided into time slots of 625 μs in length and numbered according to the master clock, wherein each time slot corresponds to an RF hop frequency, and wherein each consecutive hop corresponds to a different RF hop frequency. The nominal hop rate is 1600 hops/s. All Bluetooth devices participating in the piconet are time and hop synchronized to the channel. The slot numbering ranges from 0 to 227−1 and is cyclic with a cycle length of 227. In the time slots, master and slave devices can transmit packets. Packets transmitted by the master or the slave device may extend up to five time slots. The RF hop frequency remains fixed for the duration of packet transmission.
A master device and a slave device can be linked together by an Asynchronous Connection-Less (ACL) link for a packet-switched connection or by a Synchronous Connection Oriented (SCO) link for a circuit-switched connection. With an ACL link, data can be carried in DH (Data High rate) packets or DM (Data Medium rate) packets. The DM packets carry less data, but provide extra error protection. Every packet consists of an access code, a header and a payload, as shown in FIG. 1. The access code is used to detect the presence of a packet and to address the packet to a specific device. The access code consists of a preamble, a synchronization word and a trailer, as shown in FIG. 2. The synchronization word contains a Barker sequence to improve the autocorrelation properties of the synchronization word.
Barker sequences are widely used in radar systems, and they have been introduced as basic signals for binary phase shift keying (BPSK) and Quadrature Phase-Shift Keying (QPSK) communication, making them members of the Direct Sequence (DS) class of spread-spectrum (SS) signals. The IEEE 802.11 Standard specifies the use of Barker sequences for the chip sequence used in DSSS systems. All known Barker sequences are listed in TABLE I.
TABLE ICode Length (N)Barker Sequence11211 or 10311041110 or 110151101711100101111100010010131111100110101
The symbols in a Barker sequence are indicative of different states of a binary representation. Thus, the symbols “1” and “0” can also be shown as “+” and “−”. Hereinafter, it is preferable to use the symbols “A” and “B” to define a Barker sequence, where “A” is different from “B” in a binary representation. For example, a 7-symbol Barker sequence can be AAABBAB or BBBAABA.